1. Field of the Invention
The present invention relates to a switching regulator control circuit. Besides, the present invention relates to a switching power supply device.
2. Description of Related Art
<First Background Art and Problem>
A power transistor is used in a switching regulator, a motor driver and other various electronic apparatuses. FIG. 1 is a circuit diagram showing a structural example of a switching regulator of voltage step-up type that has a power transistor.
The switching regulator 2r includes: a control circuit 100r; an inductor L1; a rectification diode D1; and an output capacitor C1.
The control circuit 100r has a switching transistor M1 that is a power transistor; a pulse signal generation portion 10; and a driver circuit 20r. An output voltage VOUT from the switching regulator 2r is divided by resistors R1 and R2. The pulse signal generation portion 10 receives a feedback voltage VFB in accordance with the output voltage Vout from the switching regulator 2r, and generates a pulse signal SPWM whose duty ratio is adjusted such that the feedback voltage VFB nears a target level. The driver circuit 20r, based on the pulse signal SPWM, switches the switching transistor M1.
To switch on/off of the switching transistor M1, it is necessary to switch the level of a voltage (gate signal VG) of the control terminal (gate) between two values of a high level (power supply voltage) and a low level (ground level).
The driver circuit 20r is composed to be an inverter type that has a high-side transistor M2 and a low-side transistor M3. In this structure, a transition time (rise time TR) at the time the gate signal VG rises from the low level to the high level is decided depending on an electric current capability of the high-side transistor M2, while a transition time (fall time TRF) at the time the gate signal VG falls from the high level to the low level is decided depending on an electric current capability of the low-side transistor M2.
Efficiency of the switching regulator 2r becomes higher as the transition times TR and TF become shorter. On the other hand, if the transition times TR and TF become short, high-frequency electro-magnetic (EMI: Electro-Magnetic Interference) noise becomes a problem. In other words, the efficiency and the EMI are in a tradeoff relationship.
Generally, it is possible to measure electro-magnetic noise in a state only where the switching regulator 2r is mounted in a set. And, in the measurement result, in a case where the EMI regulations are not met, as an EMI measure, a set designer needs to make great efforts of modifying the control circuit 100r, the inductor L1 and a peripheral printed board and the like.
In the control circuit 100r in FIG. 1, the transition times TR and TF of the gate signal VG are decided by the capability of the transistors M2 and M3. Great efforts are required for the EMI measure, on the other hand, even in a case of being mounted in any set, the control circuit 100r is required to have universality to operate with no problem. Accordingly, the conventional control circuit 100r is designed with the transition times TR and TF prolonged such that electro-magnetic noise does not occur, so that it is necessary to sacrifice the efficiency.
<Second Background Art and Problem>
Conventionally, a switching power supply device is disclosed and proposed, which performs switching control of PWM [pulse width modulation] type in a heavy load region and switching control of PFM [pulse frequency modulation] type in a light load region.
Here, as an example of a conventional technology related to the above description, there is JP-A-2008-92712.
According to the above second background art, the efficiency of the switching power supply device is raisable in a wide load region ranging from the heavy load region to the low load region (see FIG. 11).
However, in the switching control of PFM type, a switching frequency changes, accordingly, in a case where the switching frequency overlaps a frequency band of a voice signal and a radio signal, trouble and the like occur in voice output and radio communication, whereby a risk is raised, in which performance of an application incorporating the switching power supply device is impaired.
Besides, in an application which uses a rectangular wave-shaped switch voltage, which is obtained during a process where a desired output voltage is generated from an input voltage by using the switching power-supply device, in a subsequent stage circuit (e.g., a charge pump circuit), there is a risk that operation of the subsequent stage circuit becomes unstable thanks to a change in the switching frequency, accordingly, it is hard to use the conventional technology that uses the switching control of PFM type to increase the efficiency.